1. Field of the Invention
This invention relates to a hetero-junction bipolar transistor and a method for producing the hetero-junction bipolar transistor.
2. Related Prior Art
A hetero-junction bipolar transistor (HBT) is made of group III–V compound semiconductors and shows a high current gain, a superior high-frequency performance and a high-speed switching capability. One type of the HBT has an emitter with band gap energy greater than that of a base. Another type of the HBT is a hetero junction transistor with a double hetero junction (DHBT), in which the band gap energy of the base is smaller than that of the emitter and the collector.
To enhance the high frequency performance of the HBT requires shrinking a size of the transistor. Typical DHBT has a sub-collector made of InGaAs, a collector made of InP, a base made of InGaAs, an emitter made of InP and an emitter-contact made of InGaAs, and these layers are grown on an InP substrate. When the collector made of InP is etched with an etching mask same as that used at the etching for the base layer, a size of the collector layer becomes smaller than that of the base layer because a thickness of the collector layer is in general thicker than that of the base layer and a side etching of the collector layer appears. Due to this side etching of the collector, an intrinsic collector, through which a current from the emitter flows, becomes small when making the small-sized transistor, thereby increases a collector resistance, which reduces the current gain, the high frequency performance and the high speed switching. One object of the present invention is to provide a HBT, which avoids the increasing of the collector resistance, and a method for producing the HBT.